We have investigated the possibility that differentiation of the epithelial cells in the lens to fiber cells is an apoptotic event. RNA was extracted from mouse and human lenses and subjected to RT-PCR to look for the presence of various caspases, enzymes thought to initiate apoptosis. The mRNAs for caspase 1 and 8 were present in the lens; however, repeated attempts to identify caspase 3 were not fruitful. The lenses were separated into various layers and by western blots and slot blot analyses, the presence of caspase 1 was followed. Surprisingly, the active form of this enzyme (p20) was present in all layers of the lens in both human and mouse. The p20 protein was present even in the embryonic layer from 70 year- old human lenses. In mice, p20 is found primarily in the urea- soluble fraction and is significantly decreased with cataract formation. In contrast, the p20 found in the human lens layers is primarily in the water-soluble fraction and with cortical cataract formation does not appear to be significantly lower. However, in a series of lenses from diabetic donors, there was a decrease in the presence of the p20 upon cataract formation. Analysis of the putative cleavage site associated with caspase 1 suggests certain crystallins may be the targets for this enzyme. This would support the idea that caspase 1 is involved with normal cleavage of crystallins associated with the maturation of the lens fiber cell and with lens transparency. - caspase, apoptosis, lens, differentiation